Preparation and Degradation of Rhodium and Iridium Diolefin Catalysts for the Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols

Rhodium and iridium diolefin catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols have been prepared, and their degradation has been investigated, during the study of the reactivity of the dimers [M(μ-Cl)(η4-C8H12)]2 (M = Rh (1), Ir (2)) and [M(μ-OH)(η4-C8H12)]2 (M = Rh (3), Ir (4)) with 1,3-bis(6′-methyl-2′-pyridylimino)isoindoline (HBMePHI). Complex 1 reacts with HBMePHI, in dichloromethane, to afford equilibrium mixtures of 1, the mononuclear derivative RhCl(η4-C8H12){κ1-Npy-(HBMePHI)} (5), and the binuclear species [RhCl(η4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (6). Under the same conditions, complex 2 affords the iridium counterparts IrCl(η4-C8H12){κ1-Npy-(HBMePHI)} (7) and [IrCl(η4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (8). In contrast to chloride, one of the hydroxide groups of 3 and 4 promotes the deprotonation of HBMePHI to give [M(η4-C8H12)]2(μ-OH){μ-Npy,Niso-(BMePHI)} (M = Rh (9), Ir (10)), which are efficient precatalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. In the presence of KOtBu, the [BMePHI]− ligand undergoes three different degradations: alcoholysis of an exocyclic isoindoline-N double bond, alcoholysis of a pyridyl-N bond, and opening of the five-membered ring of the isoindoline core.


■ INTRODUCTION
Ketones are a pivotal class of compounds, which can be easily transformed to diverse building blocks including (among others) imines, oximes, amines, and alkenes, the oxidation of alcohols being one of the most representative methods for their preparation. 1 Traditionally, stoichiometric amounts of chromium-and manganese-based reagents have been used for this purpose. 1a As a consequence of the large amounts of noxious waste generated, these methods have been gradually replaced by transition-metal catalysis operating under more environmentally friendly oxidants such as O 2 and H 2 O 2 . 2 In the last few years, a further step was taken with the transitionmetal-catalyzed acceptorless alcohol dehydrogenation, which does not need the use of oxidants (eq 1). The procedure displays three environmental advantages: it offers an oxidation procedure for the synthesis of carbonyl compounds, minimizing waste formation, it is a promising approach to the production of hydrogen from biomass, and it provides a direct connection with the research on hydrogen storage and transport in organic liquids. 3 The dehydrogenation of alcohols is generally endothermic at room temperature but can be performed under mild conditions, for instance refluxing toluene in open systems, since the hydrogen elimination acts as a driving force of the reaction. 4 Strongly basic media have generally been necessary for the operation of many catalysts, in particular with cationic compounds or precursors bearing halide ligands. The base cocatalyzes the dehydrogenation to generate an alkoxide, which binds to the metal and evolves into the carbonyl compound by β-hydrogen elimination. 5 To prevent the waste generated by the base, the development of precursors operating under base-free conditions is receiving great attention. 6 They coordinate ligands, being engaged in the deprotonation step. The basic center usually resides in the first metal coordination sphere 7 and sometimes in a remote position. 8 We are interested in developing catalysts for the dehydrogenation of hydrogen carriers, 9 in particular those based on organic liquids. 9f−h Thus, in the search for new precursors, some years ago we initiated a research program based on platinum-group-metal complexes and the polynitrogenated organic molecule 1,3-bis(6′-methyl-2′-pyridylimino)isoindoline (HBMePHI). 10 Previously, with a few exceptions, 11 the anion of this isoindoline had been used as a pincer ligand, which modulates the electron density of the metal center and the steric hindrance around it. 12 However, it is much more than that. We have recently reported that platinum-group-metal polyhydride complexes promote the sequential activations of bonds N−H and C−H of the isoindoline core, to afford homobinuclear and heterobinuclear compounds via mononuclear intermediates (Scheme 1). The bonding of the second metal fragment modifies the electronic structure of the polydentate ligand, which produces a noticeable perturbation of the electron density around the initial center. As a consequence of the mutual electronic influence between the metals, catalytic synergism is observed in the acceptorless and base-free dehydrogenation of secondary alcohols. The bridging ligand displays a noninnocent character, participating in the formation of the metal−alkoxide bond and in the release of molecular hydrogen. 10b These unusual findings in the chemistry of pyridyliminoisoindolines prompted us to study the behavior of HBMePHI toward the dimers [M(μ-Cl)(η 4 -C 8 H 12 )] 2 (M = Rh (1), Ir (2)) and [M(μ-OH)(η 4 -C 8 H 12 )] 2 (M = Rh (3), Ir (4)), which are cornerstones in the development of rhodium 13 and iridium 14 organometallic chemistry. This paper reports the results of this study, including the formation of novel eightmembered heterodimetallacycles and C−N bond activations in the isoindoline core, some degradation pathways of the polydentate ligand in basic medium, and the catalytic ability of some of the new complexes in the acceptorless and base-free dehydrogenation of secondary alcohols.

■ RESULTS AND DISCUSSION
Reactions with 1 and 2. The addition of 2.0 mol of HBMePHI to dichloromethane-d 2 solutions of 1 (1.0 equiv per Scheme 1. Sequential N−H and C−H Activations of the Isoindoline Core of HBMePHI Figure 1. 1 H NMR spectra as a function of the temperature of the equilibrium shown in Scheme 2: blue ■ , 5; yellow ☆, L; and black •, 1 (in CD 2 Cl 2 ). rhodium), contained in an NMR tube, produces a change in the solution color from yellow to orange. The 1 H NMR spectrum of the mixture at room temperature shows the resonances of 1 and HBMePHI (L), which appear slightly broadened, along with markedly broad signals corresponding to a new species. When the sample temperature is lowered, narrowing of all the signals is observed. At the same time, a decrease in the concentrations of both 1 and the isoindoline and an increase in the amount of a new species is clearly evident (Figure 1). Characteristic features of the new compound are 4 resonances between 4.6 and 3.3 ppm due to olefinic hydrogen atoms, which are all inequivalent, and 10 aromatic signals between 9.1 and 6.5 ppm corresponding to the CH hydrogen atoms of the coordinated ligand, which are also inequivalent. In agreement with the 1 H NMR spectrum, the 13 C{ 1 H} NMR spectrum at 213 K of the new complex displays 4 doublets ( 1 J C−Rh = 11−13 Hz) between 82 and 74 ppm for the olefinic carbon atoms and 10 aromatic signals for the coordinated isoindoline. These observations can be rationalized according to the equilibrium shown in Scheme 2, which involves the formation of the mononuclear squareplanar complex RhCl(η 4 -C 8 H 12 ){κ 1 -N py -(HBMePHI)} (5), as a result of the rupture of the chloride bridges of 1 and the coordination of the polydentate molecule to the metal center by one of the pyridyl groups. The equilibrium was studied as a function of the temperature between 293 and 223 K by integration of the olefinic resonances and the higher field aromatic signal of the free ligand. Table 1 collects the values of the equilibrium K 1 constants at each temperature. A linear least-squares analysis of ln K 1 versus 1/T ( Figure 2) provides values for ΔH°and ΔS°of −8.2 ± 0.3 kcal mol −1 and −26.4 ± 1.0 cal mol −1 K −1 , respectively.
The 1 H and 13 C{ 1 H} NMR spectra of the solutions resulting from the addition of 1.0 mol of HBMePHI per dimer to 1 in dichloromethane-d 2 show significant differences with regard to the previously mentioned spectra. Two noticeable features should be pointed out: the absence of resonances correspond-ing to the free ligand and the presence of signals due to a new compound. The latter is formed by the reaction of 1 with 5, and its concentration increases as the sample temperature is decreased. 5 it has four inequivalent olefinic hydrogen atoms. Thus, its 1 H NMR spectra contain four resonances between 4.6 and 3.4 ppm. Nevertheless, these spectra only show three complex aromatic signals in the 8.2−6.9 ppm range. These observations are consistent with the formation of an equilibrium mixture among 1, 5, and the dimer [RhCl(η 4 -C 8 H 12 )] 2 {μ-N py ,N py -(HBMePHI)} (6 in Scheme 3). The 13 C{ 1 H} NMR spectra of the mixture are strong additional evidence in favor of this equilibrium. Figure 3 shows the 13 C{ 1 H}-APT spectrum in the olefinic region, at 183 K. The equilibrium shown in Scheme 3 was also studied as a function of the temperature between 283 and 183 K. The thermodynamic parameters obtained from the values of the equilibrium constant K 2 (Table 1) are ΔH°= −5.8 ± 0.2 kcal mol −1 and ΔS°= −28.0 ± 0.7 cal mol −1 K −1 (Figure 4).
Complexes 7 and 8 were characterized by X-ray diffraction analyses. Figure 5 shows the structure of 7, whereas Figure 6 gives a view of 8. They confirm the selective coordination of the pyridyl groups of the polydentate HBMePHI molecule and the square-planar environment of the metal centers in these compounds. The coordination gives rise to Ir−N bonds of 2.124(4) Å (Ir−N(1); 7) and 2.111(6) Å (Ir−N(1); 8). These bond lengths compare well with those previously reported for other square-planar iridium(I) pyridine derivatives. 15 The 1,5cyclooctadiene ligand takes its customary "tub" conformation. The coordinated bonds display distances of 1.403(7) Å (C(21)−C(22)) and 1.428(7) Å (C(25)−C(26)) in 7 and Scheme 2. Formation of 5  Organometallics pubs.acs.org/Organometallics Article 1.413(11) Å (C(11)−C(12)) and 1.410(12) Å (C(15)− C(16)) in 8, which are longer than the C−C double bonds in the free diolefin (1.34 Å) in agreement with the usual Chatt− Dewar−Duncanson model. 16 Reactions with 3 and 4. In contrast to the chloride bridging ligand, one of the hydroxide groups of the rhodium dimer 3 is able to abstract the N−H hydrogen atom of HBMePHI. Thus, the treatment of yellow suspensions of this complex, in propan-2-ol, with 1.0 mol of the polydentate molecule for 2 h affords [Rh(η 4 -C 8 H 12 )] 2 (μ-OH){μ-N iso ,N py -(BMePHI)} (9), as a consequence of the asymmetrical coordination of the resulting anion; one pyridyl group coordinates to a rhodium atom, whereas the other metal center is bonded to the N atom of the isoindolinate core. This coordination fashion and the remaining hydroxide group give rise to a mixed double bridge, which generates an eightmembered heterodimetallacycle. Under the same conditions, complex 4 leads to the iridium counterpart [Ir(η 4 -C 8 H 12 )] 2 (μ-OH){μ-N iso ,N py -(BMePHI)} (10). The formation of 9 and 10 should take place via the intermediates (η 4 -C 8 H 12 )(OH)M(μ-OH)M{κ 1 -N py -(HBMePHI)}(η 4 -C 8 H 12 ) (M = Rh (C), Ir (D)), the hydroxo counterparts of A and B, according to Scheme 5. Similarly to 1 and 2, dimers 3 and 4 should initially undergo the rupture of a bridge, by coordination of a pyridyl group of HBMePHI to one of the metal centers. Thus, the subsequent heterolytic N−H activation of the isoindoline core by the other metal center, using the terminal hydroxide group as an internal base, would afford the mixed double bridge. Complexes 9 and 10 were isolated as orange solids in 80% and 47% yields, respectively.
The rhodium complex 9 was characterized by an X-ray diffraction analysis. The structure (Figure 7) proves the formation of the eight-membered heterodimetallacycle, which displays a boat−boat conformation 17 with the metals separated by 3.423 Å. The environment around each metal is squareplanar, as expected for rhodium(I) centers. The Rh(1)− pyridine distance of 2.1495(14) Å (Rh(1)−N(1)) is about 0.05 Å longer than the Rh(2)−isoindoline bond length of 2.1047 (14) Å (Rh(2)−N(3)), suggesting a higher nucleophilicity for the isoindoline N(3) atom than for the pyridine N(1) atom. As a consequence of this, the Rh(1)−hydroxide bond of 2.0709(12) Å (Rh(1)−O(1)) is about 0.02 Å shorter than the Rh(2)−hydroxide bond of 2.0912(12) Å (Rh(2)−O(1)). The   Figure 7, their 1 H NMR spectra display eight olefinic resonances between 5.5 and 3.0 ppm, whereas the 13 C{ 1 H} NMR spectra contain eight olefinic signals in the 85−52 ppm range. The chelate κ 2 -(N iso ,N py ) coordination is known for 1,3bis(2′-pyridylimino)isoindolate (BPHI) anions. 11 However, as far as we know, the bridge μ-(N iso ,N py ) coordination is unprecedented. Compounds bearing bridging [BPHI] − ligands are very scarce. Baird and co-workers have observed that HBPHI displaces an acetate group from Mo 2 (OAc) 4 to give Mo 2 (OAc) 3 (BPHI), with the [BPHI] − ligand bound to one molybdenum by an imino nitrogen and to the other molybdenum by the isoindoline nitrogen and a pyridyl nitrogen. 18 Broring and co-workers have reported that one of the pyridyl groups of HBMePHI undergoes a palladiumpromoted 1,3-hydrogen shift, from C to N, to afford Pd(κ 3 -N py ,N iso ,C Hpy )-pincer derivatives, which add a second palladium to the free pyridyl-imine moiety. 19 We have described the preparation of homoleptic and heteroleptic bis(osmium) complexes containing a [μ-(κ 2 -N py ,N imine ) 2 -BMePHI] − ligand, 10a whereas Li, Yang, Zhang, and co-workers have observed the same coordination fashion in an intermediate species formed in the reaction of Lu(CH 2 SiMe 3 ) 3 (thf) 2 with HBPHI to give Lu{κ 3 -mer-(BPHI)}(CH 2 SiMe 3 ) 2 . 20 Degradation of the [BMePHI] − ligand in Basic Medium. Alcohol dehydrogenation catalysts combined with bases promote borrowing-hydrogen reactions, including αalkylation of arylacetonitriles and methyl ketones. 21 The carbonyl compound resulting from the dehydrogenation process undergoes a base-catalyzed condensation with an alkyl substrate to afford an α,β-unsaturated intermediate, 22 which is subsequently reduced to the final product with the hydrogen generated in the dehydrogenation. 21 In order to explore the ability of the Rh-and Ir(BMePHI)(diolefin) systems to work in this class of catalysis, we studied the formation of 9 and 10 in the presence of a strong base.
Treatment of a suspension of 3 in propan-2-ol with 2.0 mol of HBMePHI and 3.0 equiv of KO t Bu at room temperature for 2 h leads to a mixture of 9 and [Rh(η 4 -C 8 H 12 )] 2 {μ-N iso ,N imine -   Complexes 9 and 11 were separated by using their different solubilities in propan-2-ol. Thus, complex 11 was obtained pure in 20% yield with regard to 3 as red crystals suitable for Xray diffraction analysis. Its structure (Figure 8) reveals the formation of a surprising mixed double bridge. One of the halves of the bridge is the anion resulting from the deprotonation of the NH-isoindoline group of 3-iminoisoindolin-1-one (HNC 8 H 4 NHO), which coordinates different metal centers in an N iso ,N imine fashion, whereas the other half is the azavinylidene resulting from the deprotonation of the NHimine unit of 3-isopropoxy-1H-isoindol-1-imine (HN C 8 H 4 NO i Pr). The formation of the [HNC 8 H 4 NO] − anion involves two different alcoholysis processes in the imine moieties of a [BMePHI] − ligand. The CO double bond could be the result of the substitution of a pyridylimine moiety by two isopropoxide groups. Then, the resulting diisopropylacetal intermediate 23 should lose diisopropyl ether to afford the carbonyl group. 24 In contrast, the other imino group undergoes alcoholysis of the imine−pyridyl bond. The azavinylidene bridge ([NC 8 H 4 NO i Pr] − ) arises from a similar process involving two alcoholyses on the imine functions of a second [BMePHI] − ligand. The main difference between the generation processes of both bridges is the number of molecules of propan-2-ol attacking the imine− isoindoline bond. When only a molecule of propan-2-ol attacks, the isopropoxide group remains as a substituent at the 2-position of the isoindoline core. Its steric hindrance prevents the coordination of the isoindoline-N atom, whereas the electronic difference with the carbonyl oxygen atom seems to favor the deprotonation of the imine resulting from the alcoholysis of the imine−pyridyl bond.
The rhodium atoms display square-planar coordination environments with a separation between the metals of 3.0672(5) Å, which is long for a single Rh−Rh bond. In agreement with this, overlap between their d z 2 orbitals has not been found by means of DFT calculations (M06/6-311G(d,p) &SDD(f)). Although the isoindoline coordination of the anion [HNC 8 H 4 3)) is about 0.01 Å shorter than the imine coordination to Rh(2) of 2.106(4) Å (Rh(2)−N(4)), the azavinylidene−rhodium bond lengths of 2.048(3) Å (Rh(1)−N(1)) and 2.049(3) Å (Rh(2)−N(1)) are statistically identical and similar to those reported for the complex [Rh(μ-NCPh 2 )(TFB)] 2 (TFB = tetrafluorobenzobarrelene; 2.046 (7), 2.052 (7), 2.054(6) and 2.054(7) Å). 25 The M−azavinylidene−M angle, Rh(1)− N(1)−Rh(2), of 96.93 (14)°and the distance N(1)−C(1) of 1.260(5) Å compare well with those found in other transitionmetal compounds bearing azavinylidene bridges. 26 The lengths of the coordinated CC double bonds, between 1.353(8) and 1.387(7) Å, are slightly shorter than those found in 7 and 8. The 1 H and 13 C{ 1 H} NMR spectra of 11 in benzene-d 6 at room temperature are consistent with the structure shown in Figure 8. The 1 H NMR spectrum displays a broad signal at 5.78 ppm corresponding to the imine-NH hydrogen atom and eight olefinic resonances due to the inequivalent C sp 2 -H hydrogen atoms of the diene, between 6.5 and 3.3 ppm, whereas the 13 C{ 1 H} NMR spectrum contains eight doublets ( 1 J C−Rh = 10−13 Hz) between 87 and 74 ppm, assigned to the coordinated carbon atoms. The study of the electrochemistry of binuclear complexes is always attractive due to the possible interaction between the two metals. Unfortunately, complexes 9 and 10 were unstable and decomposed in the electrode, but the cyclic voltammetry of 11 was conducted under an argon atmosphere in dry, oxygen-free dichloromethane (10 −3 M analyte concentration) containing [N n Bu 4 ]PF 6 as the supporting electrolyte (10 −1 M) and using a Ag/AgCl reference electrode (3 M, KCl). Under these conditions complex 11 displays two oxidation events, one of them quasi-reversible at 0.49 V and the second one irreversible at 1.10 V (Table 2 and Figures S1−S3). The DFT (M06/6-311G(d,p)&SDD(f)) calculations reveal that the HOMO of the complex is equally distributed between the metal centers, whereas the LUMO is located in the isoindolinate [HNC 8 H 4 NO] − ligand. The loss of one electron by each metal (two electrons per molecule) leads to the dication [11] 2+ , which also has the HOMO mainly centered on the metals, although some participation of the coordinated isoindolinate anion is observed. The subsequent loss of two electrons affords the tetracation [11] 4+ , having the HOMO mainly centered on the isoindolinate ligand ( Figure   9). These data suggest that the oxidation events are compatible with two sequential processes of two electrons: from Rh I L 2 Rh I to Rh II L 2 Rh II and from Rh II L 2 Rh II to Rh III L 2 Rh III . The successive oxidations give rise to the approach of the metal centers to 2.780 Å in the dication and to 2.747 Å in the tetracation ( Figure S6). Although these distances lie within the range of distances assumed for a Rh−Rh single bond (2.62− 2.84 Å), 27 overlapping between the d orbitals of the metals is not observed.
The Ir(BMePHI)(diolefin) systems are also unstable in basic medium. As in the rhodium case, the instability is associated with the reactivity of the [BMePHI] − ligand in basic medium, which is strongly directed by the metal center. Under the same conditions as those giving rise to the mixture of 9 and 11, dimer 4 affords a mixture of the isopropoxide dimer [Ir(μ-O i Pr)(η 4 -C 8 H 12 )] 2 and the trinuclear derivative Ir 3 (η 4 -C 8 H 12 ) 2 (κ 1 -C,η 2 -C 8 H 13 )(μ-OH)(L) (12 in Scheme 7). Using complex 10 as a reference, the L ligand of 12 can be described as the result of the oxidative addition of the C−N bond, substituted with the free pyridyl-imine group, of the fivemembered ring of the isoindoline core to the pyridyl-   Organometallics pubs.acs.org/Organometallics Article coordinated metal center. The addition of a hydride to one of the C−C double bonds of the diene coordinated to the generated iridium(III) center and the addition of an [Ir(η 4 -C 8 H 12 )] + fragment to the free pyridyl-imine group give rise to this novel molecule. The metal-promoted degradation of the five-membered heterocycle of an isoindoline is certainly notable. In this context, it should be highlighted that the isoindoline skeleton is a part of a large variety of biologically active synthetic compounds, which have a wide range of applications in medicine. 28 Complex 12 was separated from the mixture by extraction in toluene and crystallized pure in 22% yield with regard to 4 as orange crystals suitable for X-ray diffraction analysis. Its structure (Figure 10) proves the degradation of the [BMePHI] − ligand and the trinuclear nature of the complex, which is formed by an octahedral iridium(III) center (Ir (1)) and two square-planar iridium(I) centers (Ir(2) and Ir (3)). The octahedron around Ir(1) is defined by two chelates and a hydroxide-azavinylidene double bridge. The chelate C 8 H 13carbocycle coordinates with three different Ir−C distances, as expected for the κ 1 -C,η 2 -coordination, which compare well with those reported for other complexes bearing C 8 H 12 R rings similarly linked to iridium(III). 29 The σ-Ir(1)−C(21) single bond of 2.102(5) Å is about 0.06 and 0.07 Å shorter than the metal−olefin bonds Ir(1)−C (25) and Ir(1)−C(26) of 2.163(4) and 2.172(5) Å, respectively. The Ir(1)−C (21) bond is disposed trans to the pyridyl group of a (C (7),N(1))iminyl-pyridine moiety (C(21)−Ir(1)−N(1) = 169.13(15)°), which has a N(1)−Ir(1)−C(7) bite angle of 74.85(15)°. The iridium−pyridine distance of 2.236(4) Å (Ir(1)−N(1)) is slightly longer than those found in 7 and 8, whereas the Ir(1)− C(7) bond length of 1.982(4) Å is about 0.02 Å shorter than the Ir(1)−C(21) single bond and even shorter than those reported for other iridium-iminyl derivatives. 30 This suggests that, for an adequate description of the Ir(1)−C(7) bonding situation, the resonance form a shown in Chart 1 should also be taken into account. Atom C (7) is disposed trans to the hydroxide group with a C (7) (6) and 1.429(6) Å. The asymmetry of 12 is also evident in the 1 H and 13 C{ 1 H} NMR spectra. Thus, the former shows 10 olefinic resonances between 5.6 and 3.0 ppm, due to the inequivalent C sp2 H-hydrogen atoms of the carbocycles, in addition to the signals corresponding to the Ir(1)C(21)H-and OH-hydrogen atoms, which appear at 0.65 and 0.24 ppm, respectively. The 13 C{ 1 H} NMR spectrum agrees with the 1 H

Chart 1. Resonance for the Ir(1)−C(7) Bond
Organometallics pubs.acs.org/Organometallics Article NMR spectrum. Thus, it contains 10 olefinic resonances between 86.5 and 32.3 ppm. The signal corresponding to the carbocyclic C (21) atom is observed at 32.1 ppm, whereas that due to the iminyl C(7) atom appears at 198.9 ppm. This chemical shift, at an unusually low field, is more evidence for a significant contribution of the resonance form a to the Ir(1)− C (7) bond. The electrochemical behavior of binuclear complex 12 is summarized in Table 2. As shown in Figures S4 and S5, it undergoes three irreversible oxidations at 0.56, 0.74, and 1.37 V. The DFT (M06/6-311G(d,p)&SDD(f)) calculations reveal that the HOMO of the molecule is mainly located on the iridium(I) center Ir(2), whereas the LUMO is distributed along the octahedral iridium(III) center Ir(1) and its associated ligands (Figures S7−S9). After the loss of one electron, the spin density of the molecule is still localized on Ir(2) (computed spin density 0.71 e − ). Thus, it is reasonable to think that the second oxidation also takes place on this center. Thus, the peaks at 0.56 and 0.74 V can be assigned to the sequential oxidations of Ir(2), from Ir(I) to Ir(II) and from Ir(II) to Ir(III). It is likely that the third oxidation at 1.37 V could correspond to the other iridium(I) center, Ir(3). Overall, the oxidation of 12 is mainly determined by the oxidation states of the metal centers, which behave independently from each other.
Aceptorless and Base-Free Dehydrogenation of Secondary Alcohols Catalyzed by 9 and 10. The reactions were performed in toluene at 100°C, using a substrate concentration of 0.255 M and a catalyst concentration of 9.0 × 10 −3 M (i.e., 7 mol % of the metal). Table 3 collects the alcohols studied and the yield of carbonyl compounds formed as a function of the catalyst after 24 h. Ketones are obtained in moderate to high yields after 24 h. Both catalysts are more efficient for the dehydrogenation of aliphatic alcohols in comparison to that for benzylic or benzhydrylic alcohols. Thus, while ketones resulting from the dehydrogenation of substrates such as 1-phenylethanol, 3pyridylethanol, 1-(2-furyl)ethanol, and diphenylmethanol are obtained in 20−60% yield, 2-octanol and 1-cyclohexylethanol are dehydrogenated in about 70% yield. Complexes 9 and 10 are even more efficient than the binuclear polyhydrides shown in Scheme 1, (P i Pr 3 ) 2 H 2 Ir{μ-(κ 2 -N py ,N imine -BMePI-κ 2 -N imine ,C 4 iso )}IrH 2 (P i Pr 3 ) 2 and (P i Pr 3 ) 2 H 2 Ir{μ-(κ 2 -N py ,N imine -BMePI-κ 2 -N imine ,C 4 iso )}OsH 3 (P i Pr 3 ) 2 , for the dehydrogenation of aliphatic alcohols. 10b This ability is in contrast to the generally observed trend. Aromatic groups stabilize the ketone and appear to increase the dehydrogenation rate of the alcohol. The rhodium complex 9 is significantly more efficient than the iridium derivative 10 for the dehydrogenation of aromatic substrates, in particular for 3-pyridylethanol, 1-(2-furyl)ethanol, and diphenylmethanol, whereas the oxidation of aliphatic alcohols occurs with similar efficiency in the presence of both complexes.
The catalysis can be rationalized according to Scheme 8. The alcohol, which is in great excess with regard to the metal complexes, should initially displace the bridging hydroxide ligand to afford the related alkoxide derivatives [M(η 4 -C 8 H 12 )] 2 (μ-OCHRR′){μ-N iso ,N py -(BMePHI)} (E), which would be the catalytically active species of the dehydrogenation process. As in the reactions catalyzed by the polyhydrides shown in Scheme 1, 10

■ CONCLUDING REMARKS
This study has revealed that a pyridyl group of 1,3-bis(6′methyl-2′-pyridylimino)isoindoline (HBMePHI) coordinates Table 3. Metal-Mediated Acceptorless and Base-Free Dehydrogenation of a Secondary Alcohol a a Conditions: complex 9 or 10 (0.009 mmol); substrate (0.255 mmol); toluene (1 mL); heated at 100°C for 24 h. Conversions were calculated from the relative peak area integrations of the reactant and product in the GC spectra. In summary, 1,3-bis(2′-pyridylimino)isoindolinates are interesting organic anions, which can act as noninnocent bridging ligands in diverse catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. However, they should be not employed to stabilize catalysts of processes which take place in basic media, since they undergo degradation.
General Procedure for the Rh-and Ir-Catalyzed Dehydrogenation Reactions of Alcohols. A solution of the catalyst (9 or 10, 0.009 mmol) and the corresponding substrate (0.255 mmol) in toluene (1 mL) was placed in a Schlenk flask equipped with a condenser under an argon atmosphere. The mixture was stirred at 100°C for 24 h. After this time the solution was cooled to room temperature, and the progress of the reaction was monitored by GC (Agilent 6890N gas chromatograph with a flame ionization detector, using an Agilent 19091N-133 polyethylene glycol column (30 m × 250 μm × 0.25 μm thickness)). The oven conditions used are as follows: 80°C (hold 5 min) to 200°C at 15°C/min (hold 7 min), except for diphenylmethanol, 150°C (hold 5 min) to 240°C at 15°C /min (hold 13 min). The obtained values of the yield are the average of two runs. The identity of the compound was confirmed by comparison of the retention time of the product.
Computational and electrochemical data, structural analysis of complexes 7−9, 11, and 12, and NMR spectra (PDF) Cartesian coordinates of computed complexes (XYZ) Accession Codes CCDC 1913552−1913556 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.